The present invention relates to hydraulic mounts for damping vibrations.
A variety of mount assemblies are presently available to isolate vibrations. One conventional mount commonly employed to reduce vehicular vibrations is the hydraulic mount.
Conventional hydraulic mounts provide relatively low-damping, vibration-isolating characteristics (low dynamic rigidity), at vibrations of low amplitudes and high frequencies such as those generated by a running engine. These mounts also provide substantially increased-damping characteristics (high dynamic rigidity) at vibrations of high amplitudes and low frequencies such as those generated by running a vehicle on bumpy inconsistent road surfaces.
A hydraulic mount assembly of prior invention includes a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. This cavity is partitioned by a plate into two chambers that are in fluid communication with each other through a relatively large decoupler orifice in the plate. A primary chamber is formed between the plate and the hollow rubber body. A secondary chamber is formed between the plate and the diaphragm.
A decoupler is positioned in the plate's decoupler orifice and reciprocates in response to vibrations so as to produce small volume changes in the two chambers. When, for example, the decoupler moves toward the diaphragm, the volume of the primary chamber increases and the volume of the secondary chamber decreases. In this way, at certain low vibratory amplitudes the major fluid flow is through the decoupler orifice so that the mount exhibits low dynamic rigidity to isolate engine vibrations and hydraulic damping is not provided.
In addition to the decoupler orifice, a smaller orifice track is provided which extends around the perimeter of the plate so as to have a large length-to-diameter ratio. Each end of the orifice track has an opening; one opening communicates with the primary chamber and the other with the secondary chamber. The orifice track provides the hydraulic mount assembly with a means of providing hydraulic damping for high dynamic rigidity at high amplitude vibrations where the decoupler operates to close the decoupler orifice. When combined, the oscillating decoupler and the orifice track provide at least two distinct dynamic modes of operation. The operating mode is primarily determined by the flow path of the fluid between the two chambers through either the decoupler orifice or the orifice track.
More specifically, small amplitude vibrating inputs, such as from the engine or the like, are isolated by the mount which exhibits low dynamic rigidity due to decoupling wherein, the decoupler floats as fluid exchange occurs through the decoupler orifice, as described above. On the other hand, large amplitude vibrating inputs force the decoupler against a plate closing the decoupler orifice to produce high velocity fluid flow through the orifice track, and accordingly, a high level of vibration damping force and high dynamic rigidity. In each instance, as the decoupler moves from one seated position to another, a relatively limited amount of fluid can bypass the orifice track by moving around the sides of the decoupler to provide limited help in smoothing the transition between the operational modes.
When fluid flows through a decoupler orifice or orifice track, flow rate is maximized at a specific frequency due to resonance. This resonance frequency is dependent on the flow area and the length of the flow path provided. A typical mount is designed to employ this physical characteristic to provide both low dynamic rigidity and high dynamic rigidity modes of operation at separate selected frequencies.
While the two distinct modes of operation provided by conventional hydraulic mounts provide generally satisfactory operation, they are not sufficient to furnish the desired maximum damping and noise suppression under all the continuously varying conditions encountered during vehicle operation. A noise problem is experienced as the decoupler moves during abrupt load changes and the accompanying mode transitions.